http://www.sublimetext.com/docs/3/packages.htmlOverriding Files From a Zipped Package

To override a file in an existing package, just create a file with the same name under the Packages/<Package Name> directory.

For example to override the file function.sublime-snippet in the Python.sublime-package package that ships with Sublime Text, create a directory called Python under the <data_path>/Packages directory, and place your function.sublime-snippet file there.

If I create an empty file ~/Library/Application Support/Sublime Text 3/Packages/PHP/define(-).sublime-snippet ST3 crashes upon start. If I copy the snippet from the ZIP file and change the tab trigger to something else, now both the trigger I don't like in the first place and the new trigger work.

You are right, I now got it to work, it seems that I had another copy of the package in the folder. The quick solution for me now was to change the scope for each snippet to something that doesn't apply to PHP. Thanks!